Try this Drag the orange dots on any vertex
to reshape the triangle. Notice the location of the orthocenter.

The altitude of a triangle (in the sense it used here) is a line which passes through a
vertex
of the triangle and is perpendicular to the opposite side. There are therefore three altitudes possible, one from each vertex.
See Altitude definition.

It turns out that all three altitudes always intersect at the same point - the so-called orthocenter of the triangle.

The orthocenter is not always inside the triangle. If the triangle is
obtuse, it will be outside.
To make this happen the altitude lines have to be extended so they cross. Adjust the figure above and create a triangle where the orthocenter is outside the triangle.
Follow each line and convince yourself that the three altitudes, when extended the right way, do in fact intersect at the orthocenter.

Summary of triangle centers

There are many types of triangle centers. Below are four of the most common.

The Euler line - an interesting fact

It turns out that the orthocenter, centroid, and circumcenter of any triangle are
collinear
- that is,
they always lie on the same straight line called the Euler line, named after its discoverer.
For more, and an interactive demonstration see Euler line definition.